Szilard



L. SZILARD Sept. 10, 1963 REACTOR 13 Sheets-Sheet 2 Filed Sept. '20, 1946 Sept. 10, 1963 L. szlLARD 3103475 REACTOR Filed Sept. 20, 1946 15 Sheets-Sheet 3 l.. szlLARD AREACTOR sept. 1o, 1963 Filed Sept. 20, 1946 l 13 Sheets-Sheet 4 FUE 5 Wwmjwgf Sept 10, 1963 v L. szlLARD 3,103,475

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W J-for ne L. SZILARD Sept. 10, 1963 REACTOR 13 Sheets-Sheet 9 Filed Sept. 20. 1946 L. szlLARD REACTOR 13 Sheets-Sheet 1.0

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IN VEN TOR. Zl/z'ness 4b. BY Le@ 5j la rd wam/wf w United States Patent O 3,103,475 REACTOR Leo Szlard, Chicago, Ill., assignor to the United States of America as represented by the lUnited States Atomic Energy Commission Filed Sept. 20, 1946, Ser. No. 698,334 3 Claims. (Cl. 204-193.2)

This invention relates to nuclear physics and more particularly `to a novel method and means for establishing and controlling Ia fast neutron nuclear ssion chain reaction.

It is known in the art that a controlled slow neutron chain reaction may be established by combining iissionable material and neutron moderator to form a reactive mass, the moderator being effective to reduce the average neutron energy to a value at which the fission cross section of said material is relatively great.

In slow neutron reactors a neutron iissionable isotope such as U233, U235, or Pu239 or mixtures thereof is subjected to fission by absorption of neutrons and a selfsustaining chain reaction is established by the neutrons evolved by the iission. In general, such reactors comprise bodies of compositions containing iissionable material such, for example, as natural uranium, disposed in a neutron slowing material which slows the neutrons to thermal energies. Such a slowing material is termed a neutron moderator. Carbon and D20 (heavy Water) are typical moderators suitable for such use. Heat is evolved during the reaction which Iis removed by passage of a coolant through the reactor or in heat exchange relationship therewith. Specific details of the theory and essential characteristics of such reactors are set for-th in U.S. Patent `2,708,656 of Enrico Fermi and Leo Szilard, Serial No. 568,904, filed December 19, 1944.

In reactors of the above-described type a substantial portion of neutrons is lost to the chain reaction by` absorption in noniissionable material and by absorption in iissionable `material Without fission thereof. Consequently the amount of noniissionable material in the reactor must be carefully limited. This practice'has been disadvantageous, inasmuch as it has required limitation of the amount of coolant, the number of cooling conduits, and the quantity of other neutron absorbent structural components, thereby imposing limitations upon the power output of the reactor and upon the structural quality thereof. Moreover, the use yof nontissionable neutron absorbent coolants and other neutron absorbent equipment within the reactor has resulted in a serious loss -of neutrons which might otherwise have been available for effecting further iissions or producing ssionable isotopes from noniissionable isotopes, as hereinafter discussed.

In -accordance with the present invention it has been found that neutron absorption losses may be` greatly minimized by establishing and maintaining a self-sustaining nuclear iission chain reaction while avoiding the slowing of evolved neutrons below an average energy of about 1,000 ev., and preferably below about 10,000 ev. At such high energies, it has been discovered that the elements of 'atomic -number of 11 to 83, which are generally used as structural, cooling, or other elements in a reactor, have vneutron absorption cross sections which are substantially lower than their absorption cross sections for neutrons at thermal energies. In contrast, the absorption cross section for neutrons at high energies of certain heavy isotopes, such as Th2?2 or U23", which are converted to thermally issi-onable isotopes upon neutron bombardment, are not so greatly reduced; land hence .their tendency for neutron absorption is correspondingly higher than for lower atomic number elements. Thus, a substantial sav- 3,103,475 Patented Sept. l0, 1963 ICC ing of neutrons may be effected by maintenance of the high energy level.

Similar advantages may accrue by -operating neutrom'c reactors at lower energies, as for example, as low las l0 ev. or even as low as 0.3 ev., which energies are subs-tantially above the energy of thermal neutrons at room ternperature, that is `about 0.03 ev. However, the higher energizes -of 1,000 ev. and above are preferred, inasmuch as nonrnoderating neutron reflectors may be utilized with reactors operating at these values. For example, as hereinafter discussed in detail, the isotopes U238 and Th232 may be utilized for this purpose, particularly in view of the fact that whatever neutron absorption occurs in these isotopes resul-ts in the production of thermally iissionable isotopes, as for example, 94239 from '0238 and U233 from Than In neutronic reactors, the ratio of the fast neutrons produced in one generation by the iissions to the original number of fast neutrons in 1a theoretical system of innite size where there can be no external loss of neutrons is called the reproduction or multiplication factor or constant of the system, and is denoted by the symbol K. For any iinite system, someV neutrons will escape from the periphery of the system. Consequently, a system of finite size may be -said to have a K constant, even though the value thereof would only exist if the system as built were extended to iniinity without change of geometry or materials. Thus, when K is referred to herein as la constant of a system of practical size, it always refers to what would exist in the same type of system lof iniinite size. If K can be made sufficiently greater than unity to` indicate a net gain in neutrons in the theoretical system of infinite size, and then an lactual system is built to be sufficiently large so that this gain is not entirely lost by leakage from the exterior surface of the system, then a self-sustaining chain reacting system of finite and practical size can be built to produce power and related lay-products by nuclear iission of natural uranium. The neutron reproduction ratio in a system of finite size, therefore, differs from K by the external leakage factor, and by a factor due to the neutron absorption by localized neutron absorber, and the reproduction ratio, R, must still ,be suliiciently greater than unity to permit the neutron. density to rise exponentially with time in the system as built. Y

During the interchange of neutrons in a neutron-moderated system of iinite size, comprising bodies of any size disposed in a neutron moderator, neutrons may be lost to the chain reaction in four ways: by absorption or capture in the uranium content of the bodies without producing ssion; by labsorption or capture 4in the moderator material; by absorption or capture by the impurities present in both the uranium :bodies and the moderator; and by leakage out of the system through the periphery thereof. In a fast reactor all of these losses, except the last, are either substantially eliminated or are `greatly reduced.

It has been ldiscovered that stabilization of a fast neutron' chain reaction is possible in View of the fact that la potentially chain reacting mass of this type has two delinite `critical dimensions, a smaller one at Which the total neutron emission, including the delayed neutron emission, is capable of sustaining a reaction, and aplarger critical dimension at which the instantaneous neutron emission alone is suflicient to sustain a chain reaction. If the chain `reacting mass or reactor is operated at a size slightly greater than the smaller of these two values and considerably smaller than the larger of these twovalues (neutron reproduction ratio greater than 1.0 but not above about 1.01` or 1 plus Athe fraction of neutrons evolved which :are deliayed neutrons), the intensity of the chain reaction rises exponentially with time at a rate which is practically independent of the life time of the fast neutrons in the chain reacting m-ass and is determined by the time constant of the delayed neutron emis-` sion. Thus, according to the present invention, la fast neutron 'chain reaction may be controlled within the delayed neutron range, just as a slow neutron reactor is controlled, as is more fully discussed i-n the previously. mentioned copending application. However, special means for effecting such control may be provided as hereirl-after set forth.

Several advantages accrue from the operation of a neutronic reactor at high neutron energy values. For eX- ample, the-poisoning elect of impurities or poisons, such as Xenon135, formed with-in the reactor as fission products areV eliminated, thus Veliminating the necessityof building excess reactivity into the reactor to accommodate compensationV for such poisons by the removal of shims during the reaction, :as more fully discussed in the abovementioned copending application.

Another advantage of yfast neutron reactors is their small size and compactness resulting from the substantial elimination of neutron moderator, andalso the elimination' of substantial neutron losses due toabsorption by thecoolant andother impurities, as previously discussed. It may be noted in this connection that by decreasing neutron' absorption in the coolant, the handling of the coolant land of the associated circulating apparatus is simplified; lan'd serious hazards to operating Ipersonnel, who may come into contact with the coolant and the associated-circulating apparatus, fare substantially reduced.

Another advantage of a fast neutron reactor is anV increase in the available number of neutrons per ssion inther'eactor. Ithas :been found that such lan increase is attained in a fast neutron reactor such as abovedescnibed, wherein the issionable material is' Pu2-39, and it is believed that the increase is due to the fact Athat the average neutron energy is greater than 0.3 ev., at which energy maximum resonance absorption occurs in thisisotope without tissioning` thereof and also may be due to the fact that more neutrons appear to be emitted on the average-for each fissionI of a P11239 atom by a fast neutronthan byy a slow neutron.

It will be understood that4 the vl,abover features simplify the construction of a so-called breeder system, wherein a chain reaction is utilized to produce lissionable material at a rate greater than the rate'of consumption of iission'able material-within the chain reactingr composition. Thisisaccomplished by neturon bombardment of fertile material adapted to undergo nuclear reaction productive of ssionable material, as hereinafter discussed. I-n this connection it has been found that neutron losses in la slow neutron reactor utilized in' a breeder system are frequently so great as to reduce the rate of production of ssionable material to a value lower than that at which 'ssionable material is consumed, thereby preventing the use `of suchV a system for the breeding of ssionable material.

f A general object ofthe invention is, therefore, to establish and control a fast neutron chain reaction wherein little or Vno neutron moderator is provided to slow down the neturons which tale part in the chain reaction. A preferred' embodiment of the novel system, as abovenoted, operatesV at an' average neutron energy 'above 1,000 ev. and preferably within or above the range from 10,000 ev. to 100,000 ev. If desired, an average neutron energy lsubstantially labove thermal energies (at least 0.3 ev.) may be maintained with beneficial results, as above discussed.

Another object of the invention is to establish a controllable Vfast neutron chain reaction by so designing the reactive composition that the total neutron emission therein, including delayed'neutrons, is suliicientto sustain a chain reaction, the instantaneous vneutron emission'` alone beinginsutiicient tofsustain la chain reaction. g j

Stillanother object of the invention is to provide a neutron chainv reaction to Ibombard isotopes capable'ofV novel method and'means for controlling a fast neutron chlain reaction Vby introducing neutron moderator into the reactivecomposition and withdrawing said moderator therefrom to regulate the neutron reproduction ratio of the composition, as hereinafter more fully described.

A further object of the invention is to provide a novel method andv means for breeding fissiona'ble material or other desired isotopes by utilizing the neutrons from a fast undergoing nuclear reaction productive, of thermally' -ssionable material.Y

The foregoing and other objects and `advantages of the invention will become apparent vfrom a consideration of the following specification andthe accompanying drawings, wherein:

FIG. 1 is a diagrammatic illustration of a -fastneutron reactor embodying the invention, portions of the structure being shown in central vertical cross section and other portions being shown in side elevation;

FIG. 2 is a cross sectional view` taken on the line 2--2 of FIG. l; Y

FIG. 3 is an enlargedy vertical sectional view through one of the reactive rods shown in FIGS, l and 2;

FIGS. 4 and 5 illustrate another embodiment ofthe invention, FIG. 4 beingy a yfragmentary vertical sectional view partly in elevation of a reactor 'similar to that shown in FIGS. l and 2; and FIG. 5 being a sectional view taken on the line 5 5 of FIG. 4; 1 v PIG. 6 is a side view of `another embodiment fof the invention, portions of this view'being, shown in vertical section kand other portions being shown inside elevation;

FIG. 7. is a fragmentary side elevation taken from the left as seen inFIG. `6, portions of the structure-being shown in vertical section; 1

FIG. 8 is an enlarged view of a portion of FIG. 7; Y 1

FIG. 9 is an enlarged cross sectional view taken on the line 7-7 of FIG. 6;'and

FIGS. 10 and 1l are fragmentary verticali-sectional views illustrating, respectively, a r-eactiverrod and a reilector rod shown in FIGS. 6 and 9;

FIGS. 1'2 to 17 inclusive are,V graphs representingf cal-V culationswhich may be utilized in the construction of fast neutron reactors without neutron reflectors. Values of the Laplacian indicated in these tigures are Vnegative values. l Descn'bingthe invention in detail and referring rst to FIGS.V 1 and 2, the embodiment of the invention illustrated therein comprises` a reactorgenerally designated Z in the form.r of a plurality of reactive or issionable rods4, the composition and construction of which are hereinafter discussed. The rods, only a -few of which are shown, are disposed within a cylindrical tank or containerrj pref,- erably formed of steel and provided with -top and-.bottom finletand outlet conduits 8 and 10 (FIG. 1) respectively, adapted to. convey` a coolant, such .as helium -or a liquid metal such as liquid sodium, liquidl `sodium potassium alloys or other suitable substance having an atomic number above 10, Iwhich lis. relatively .inelective to moderate neutrons within the reactor. v

Therods 4-are supported by a steel plate '12 which is seated on ribs '14, carried by 4the bottom Wall of the tank 6. The plate 12 also, abutsthe 4bottom edge of a steel liner 1-6 and comprises one or more openings or ports 18.1communicating with passages 20' in said liner to accommodate ilow of thecoolant from the tank- 6 into the before.-

mentioned outlet conduit 1-0. The upper ends of the rods 4 are supported by a steel plate 22 mounted on steelpedes-- tals or columns 24v carried by the plate 12. One of the rods -V 4 is shownin detail in FIGA. 3 andY comprises a Vcylindrical member 26,]containing thermally iissionablewrnaterialj en-.

closedy within a thin walled steel V.tube Vor 1sheath 28 closedn of a. Steel pin Sesam-pins. beine Vadapted for 'engagement Y with complementary openings in the plates 12 and 22, as best `seen in FIG. 1.

The rods 4, as hereinafter discussed in detail, contain sufficient thermally iissionable material to sustain a fast neutron chain reaction. Preferably, the rods 4 contain one or more isotopes herein called fertile isotopes or material, lwhich `are fissionable by fast neutrons and are substantially noufissiona'ble by slow neutrons, and which absorb or capture neutrons, fast or slow, to undergo nuclear reaction productive of ssionable material, as for example, the isotopes Th232 and U239, which are converted to U233 and Pu239, respectively, by nuclear reaction under neutron bombardment. The rods also initially contain one or more isotopes issionable by slow or fast neutrons, such as fissionable U233, U235, or Pu239 or other isotopes, herein called thermally7 fissionable isotopes or material, inasmuch as they are tissionable by thermal neutrons which `are neutrons having velocities of -about 2,200 meters per second. Such thermally issionable isotopes or materials are also vfissionable with higher energy neutrons. Preferably, the fertile material and the thermally fissionable material -are so proportioned and disposed that thermally tissionable material is produced during the reaction at a rate greater than that at which said thermally fissionable material'is consumed or fissioned, so that the system above-described is a breeder system. For example, assuming that the rods 4 .are formed of an alloy of U238 and P11239, about 3 neutrons are emitted on the average for each fission of a P11239 atom under the action of a -fast neutron. Of these neutrons, one is needed to sustain the chain reaction, and a fraction of a neutron, on the average, is lost to the system as above-described. The remaining neutrons are captured by the U239 atoms or cause iission thereof, resulting in about 2.3 neutrons on the average for each fission. By eliminating neutron moderator, a relatively great numberof neutrons in the system are above the fission threshold (900,000 ev.) of Um, and thus by ssioning the same increase the amount of neutrons available for the production of P11239 by absorption in U228.

In this embodiment the chain reaction may be controlled by various methods such as by introducing into the chain reacting mass or reactor 2 at the center or core thereofa quantity of liquid, such as molten Ilead or a lea-d bismuth alloy capable of slowing the neutrons to awlimited extent. The core of the reactor 2 is generally designated as the portion constituting about one-eighth thereof closest to its center, Where the neutron density is normally greatest. Thus, the reactor can be controlled by.varying the :amount of neutron slowing liquid in the core up to about one-fifth of the mass thereof. The neutron slowing liquid is carried by steel tubes 3S connected to inlet and outlet lines 40 and 42. The inlet line 40 is connected fto 4a discharge side of a pump 44, the suction side of which is connected to a tank or sump 46 containing the moderating liquid, which is maintained at a predetermined minimum temperature value by `any suitablemeans (not shown). The tank 46 is also connected -to the outlet line 4Z.

The inlet line 40 is connected to a by-pass line 48, which is connected to the inlet side of a conventional heat exchanger 50, the outlet side of Which is connected to the suction side of the pump by a line 5l. Thus, a portion of the neutron slowing liquid is constantly reduced to a minimum temperature value Within the heat eX- changer 50 and is passed through the pump during operation thereof to prevent excessive heating thereof due to heat absorbed in the moderating liquid Within the reactor 2.

The preferred method of controlling the chain reaction is to construct the chain reactor to a size slightly smaller than the critical size with the tubes `38 sub- 4stantially empty. 'then as the moderator liquid is pumped into the tubes 38, the reproduction ratio of the reactor is increased to a value above unity inasmuch Yas the fission cross section of the thermally t'issionable material is ygenerally inversely proportional to fthe energy of the neutrons within the reactor.

In such an arrangement, the rods may be spaced to provide any `desirable volume ratio between the coolant `and the thermally iissionable material lwhere a coolant such as lbismuth is utilized which is capable of moderating neutrons to a limited extent. Thus the average neutrol energy may be maintained at .any desired value above about 0.3 ev. and preferably iabove 1000 ev.

It may be noted that experiments have `shown that the average number of neutrons emitted per fission of a U238 atom in fast neutron reactors is about 1.2 in a composition composed principally of Pu239 and U23". Funthenmore, U299 undergoes an n, 2n reaction which may afford a .2/2 percent increase in the number of neutrons produced Within the reaction zone. This n, 2n reaction produces 93 Np237 as follows:

inasmuch as 93323'7 is fissionable with a fission cross section which has been measured as 0.018i0-005 barns for thermal neutrons, this isotope may participate to some extent in the cha-in reaction.

A number of neutrons in the nuclear fission chain reaction are yabsorbed by Pu239 atoms .to produce Pu240, an isotope fwhich has not yet -been measured fwith respect to its fission cross section. However, assuming that this isotope is not fissionable, the production thereof is generally inversely proportional to the average neutron energy in the reactor, inasmuch as the absorption (without fission) cross section of P11239 decreases to a greater extent than the fission cross section thereof 'as the average neutron energy increases. For this reason, theproduction ratio of P11240 can be counted upon to `decrease by Ia factor of vabout 3 as the average neutron energy is increased from thermal energies to about 1 mev. Thus, in a mixture of Pu239 and U239, at least about 2.5 neutrons on the average `are emitted per iission of a Pu239 atom. This ligure may be multiplied by l.2, the average number of neutrons emitted per fission of a U233 atom, thus giving a value of about 3 neutrons emitted per thermally fissionable Iatom destroyed, thereby making possible a gain of about one thermally fissionable atom per similar atom destroyed. it may be noted, however, that according to certain experiments about 3 neutrons `are emitted on the average for each fission of a Pu239 atom, a value which would afford an even greater breeding gain of thermally ssionable material.

FIG. l2 comprises a plot of the ratio of the Laplacian (A) to the square of the mean density of the reactor as a function of the concentration of U235 in a reactive metal composition comprising U238 and U235 in a pile or reactor such as that shown in FIGS. 1 to 3, the density being defined as the weight of the designated materi-al per unit volume of the chain reacting zone. i Two curves are plotted, one for :an :assumed mean neutron fission energy of 1.0 mev., and the other for an assumed energy of 2.0 mev. In this connection, it will be understood that the critical size of a pile may be calculated by the following formulae, if the Laplacian is known, as is more fully discussed in the above-mentioned Fermi and Szilard application. l

In case the reactor is'to be built in the form of ya large spherical structure, the critical radius (R) is given by In case the structure is to be 4a rectangular` parallelopiped -with sides al, a2 and a3 thecritical size is given by thegfonnula n ignored that in FIG. 12, the

Laplacian Ratio is'plotted along the Vordi-nate of the graph, and the ratio of U23 atoms to'U235 atoms is plotted along the `abscissa ofv the graph. According to this graph, the Laplacian becomes very low for zero amounts of U238, a phenomenon which is probably due to the neutron reflecting or scattering effect of U28, which moderates neutrons to a limited extent by inelastic collision therewith, thus reducing the average neutron energy and consequently increasing the iission cross section of U235. Y

Referring now to the graph of FIG, 13, there are shown two, curves indicating respectively critical pile mass in kilograms and critical mass in kilograms of U235 for a minimum cylindrical pile i.e., one in Vwhich H is equal to 1.847R, such as that shown in FIGS. 1 to 3, kand another vcurve indicating the Laplacian for a pile of any con-tour, the mean pile density being assumed t-o be 14.2 grams per cubic centimeter in the derivation of all three curves. All of the curves shown in FIG. 13 are derived, trom the lower curve of FIG. 12, for which the vaverage neutron energy, Ef, is equal to 2 mev. For example, according to this graph, assuming that the ratio of VU238 atoms to U235 atoms in metallic state is 1 to 1, approximately 250 kilograms of U235 metal would be requiredin a cylindrical pile with a critical mass of 500 kilograms, assuming that the pile is composed solely of H238 and UZ35 metal with the above-mentioned reactorV density.' The Laplacian of such a pile is 0.0264. In order'lfor la pilegof this'type (FIGS. 1 to 3) to breed, the concentration of U235 is preferably not greater than about 1.ato-n1 of U235 to 1.9` atoms of Um, although this depends materially upon the mean energy of the neutrons, as above discussed. FIG. 14 shows the eiiect upon the lLaplacian of changes in reactor density of a tast neutron pile -for a 20 percent concentration of -U235 and 331/3 percent concentration thereof.` The two curves shown in FIG. 14 -are both derived from the upper curve of FIG. l2, for which the -average neutron energy, Ef, is equal to 1 mev.

Referring now to FIG. 15 the Laplacian of a U235O2 fast neutron pile is plotted as a function of the reactor density. VThe curve :at the left of this ligure is based on 1an assumed uranium scattering cross-section of `10 barns -(1 1barn=10r24cm.2), and the curve at the lower part of this figure is based on an assumed uranium scattering cross section of zero. It will be understood that inV actual practice, reactors will -fall within the area Ibetween these two curves. Thus if the laverage yamount of U02 in the 'reactor is 40() pounds per cubic foot of the reactor, the Laplacian will fall bet-Ween 0.005 and 0.025. l

Referring now to FIG. 16, the critical radius of a U235O2v cylindrical pile is plotted as a function of the average reactor( density,l the height ,-of the pile being 1.847 times the radius. In'this graph, as in the gnaphof FIG. 13, two curves are shown based respectively on a uranium scattering cross section of 10 barns, yand a uraman scattering cross section 'of zero. Thus assuming a Vreactive zone wherein the reactor has van average density of approximately 200 pounds per cubic foot, the critical radius of such a reactor .fwould be between Vabout 8 `1.25 feet, and about 2.75. feet depending upon the actual scattering cross section of the uranium. I-f on the` other hand, the Idensity of the uranium-oxide were about 450 .pounds per cubic foot, the critical radius would be between 0.5 feet and 1.25 feet depending upon the actual scattering cross section of the uranium.

FIG. 17 Iis fa graph illustrating the criticalmass'of U02 in a minimum cylinder, two curves being illustrated based respectively on a uranium scattering cross section of 10 barns and a uranium scattering cross section of zero. Thus, for example, if the concentration of uranium oxide in the reactor is 4about 200 pounds per cubic foot of the reactor, the critical mass of the reactor would be be- .tween 2,500 pounds and 25,000 pounds -depen'dingrnpon the actual scattering cross section of uranium.

The embodiment shown in FIGS. 4 andk 5 comprises a reaction tank 54 with coolant inlet and outlet conduits 56 and S8 as in the previously described embodiment. The tank 54 contains a stainless steel liner 59 substantially identi-cal with that shown in FIG. 1. A steel tube or conduit 60 extends through the tank 54 'and through the inlet 4and outlet conduits 56 and 58 to accommo-V date ilow of a neutron absorbing or moderating flu-id, such as mercury or liquid bismuth for the purposefof controlling the reaction, as discussed m connection with FIGS. 1 to 3.

The reactor shown in FIGS. 4 and 5 comprises a plu# rality of xed neutronically pure uranium rods 61 supported by top and bottom plates or Walls 62 and 64 asin the previously described embodiment, the rods. Within the area indicated at X (FIG. 5) being initially enriched with a thermally issionable isotope, such as Pu239.

A plurality of pairs ot rotatable support plates 66 and 68 are disposed within the tank 54, the bottom platesV 68 being rotatably supported from the before-mentioned wall 64, and theV top plates 66 being connected to actuating rods 70V for rotation thereby. Connected between each pair of plates 66 and 68 are a plurality of neutronicallypure uranium rods .72 VVconstituting a rotatable bundle or group of rods participating inthe nuclear iis-V In this arrangement, the rods 72" sion chain reaction. disposed within the area X are initially enriched with a thermally issionable isotope.V As the fast neutron chain reaction breeds additional thermally iissionable materialv in the various rods 61 and 72 thus increasing the neutron reproduction ratio of the system, the bundles of rods 72 are rotated to bring the unenriched rods 72 into the area X which is the core of'the reactor at which. the neutron density is greatest. Thus, the natural uranium rods 72 which are initially disposed externally of the area X are gradually moved into this area to berapidly enriched in thermally iissionable material by conversion of the U233 atoms under intense neutron bombardment to Pu239 atomsfand at the same time fthe tendency of Vthe neutron reproduction ratio to rise is temporarily checked or controlled as the natural uranium rods 72`fare moved into the area X, until all of the natural uranium rods 72 have been enriched, at which time the reactor may be disassembled for the purpose of recovering the thermally issionable material produced as above described.

In the arrangement disclosed in FIGS. 4 and 5,`theenY t S8 is'preferably a molten bismuth-leadVV alloy which oc-V cupies an area within the tank 54 equal to about one-third of the enriched'core X, saidtalloy passing through the reactor ata velocity ofabout 15 meters per second. The

reactive rods 61 and V72 are preferably about one-half centimeter' indiametergoperating atla' Vtemperature of about 700 C; at the center of the central rods, thus accommodating a temperature increase of aboutV C. in the coolant. Under these conditions about 250,000 kw. will be removed from the reactor. Y Y q Referring now to FIG. 6, the system shown therein comprises inner and outer steel tanks 102 and 104, the inner tank containing a plurality of composite rods 106 and the outer tank containing a plurality of composite rods 108, all of said rods being supported, as hereinafter described in detail, from a biological shield y110 composed of any suitable material adapted to absorb biologically harmful emanations, such as neutron and alpha, beta, and gamma rays.

The shield 110 is supported by lingers 112 connected to I beams 114 as by bolts 116. The beams are mounted Within a biological shield 118 with a central opening 120 accommodating the before-mentioned shield 110. The top of the opening 120 is closed by a cover plate 122, Which may be removed to accommodate assembly and disassembly of the rods 106 and 108.

One of the rods 106 is shown in detail in FIG. 10 and comprises a cylindrical segment 124i composed of thermally iissionable material disposed between cylindrical segments `126 and 128 composed principally of fertile material. The segment y124 is connected to the segments 126 and 128 by steel couplings 130 and 132, respectively, the coupling 130 being provided with spaced fins 1311 adapted to center the rod 106 in an opening through a wall or partition 134 within the tank 102. The segment 126 isconnected to a cylindrical beryllium segment 136 by a coupling 138 formed with ns 140 adapted to center the rod '106 in an opening within a Wall 142 of the tank 102. The beryllium segment 136 is connected to an iron segment 144, which is, in turn, connected to another beryllium segment r.146. All of the segments are closed within thin walled tubes or sheaths 148 adapted to space the segments from a coolant circulated through the system, as hereinafter described, for the purpose of absorbing the heat of the nuclear fission chain reaction.

The rod 106 is supported from the shield 110 by a spring 150 bearing against a cap 152 connected by a pin or spindle 154 to the top beryllium segment 146, said spring being seated against a spring plate 158 bearing against the top of the shield :110. 'Ihe cap carries a sleeve 160 enclosing the spring 150, which functions to place the rod 106 under tension, inasmuch as the bottom thereof is interlocked with the bottom wall 162 of the tank 102. Thus, any tendency of the rod to warp due to the heat of the reaction is resisted by the tension on the rod.

The rods y106 are spaced apart .046 inch at their closest portions and are disposed about .494 inch apart, center to center. inch, and the outside diameter of the jacketed segment is about .448 inch. One-hundred and sixty nine active rods 106 may be utilized in an arrangement wherein the thermally fissionable material is substantially pure U235. The inner tank 102, which is hexagonal is shown in FIG. 9, and is about 6.688 inches in Width between parallel sides thereof.

One of the rods 108 is shown in detail in FIG. ll and is substantially identical with the rod 106, except for the larger diameter of Ithe rod 108 and the elimination of the thermally ifissionable segment 124. The portions of the rod 108 (FIG. ll) corresponding -to those of the rod 106 (FIG. l0) are identified by corresponding numerals, and it will be seen that the bottom Isegment 120 of the rod 108 containing fertile material is longer than the corresponding segment 128` of the rod 106 because of the elimination of the thermally lissionable segment 124 from the rod S. The rods 108, as laboveanoted, are contained within the tank 104 which has .an inner diameter of about l5.875 inches.

The rods 108 'are preferably about .875 inch in diameter and they are spaced apart about .916 inch center to center, thus providing a space of about .041 inch between the closest portions of adjacent rods. The jackets 148 of the rods 108 yas well as the rods 106 have a wall thickness of .027 inch. In the instant embodiment of the invention one-'hundred and ninety-two rods 10S are utilized.

The rods 106 and 108 are cooled by any suitable coolant such as molten bismuth or a sodium potassium alloy containing, for example, percent by weight of sodium and 50 percent by weight of potassium circulated, as indicated by the arrows in FIG. 6, by inlet and outlet conduits 109 and 111. The inlet conduit is connected to the outer tank 104 and the coolant llows downwardly therethrough into the tank 102 through ports at the bottom thereof |and upwardly therethrough into the ou-tlet conduit 111. Thus, the coolant absorbs the heat of the nuclear fission chain reaction, and the energy in the form of heat carried from the reactor by the coolant may be utilized for power or other purposes, if desired.

Referring again to FIG. 6, it will be seen that the thermally iissionable segments 124 of the rods 106 constitute a reactive portion indicated at A, wherein the nuclear chain reaction -is sustained, said reactive portion being entirely surrounded by fertile material within the rods 106 and 108. This blanket of fertile material surrounding the reactor A not only absorbs neutrons to produce thermally ssionable material, as `above discussed, but also functions as a fast neutron reflector, which reilects or scatters fast neutrons escaping from the reactor A by inelastic collision with the atoms of fertile material, thus reflecting the escaping neutrons back into the reactor to -diminish neutron losses from the periphery thereof, thereby decreasing the critical size at which said reactor is capable of sustaining `a chain reaction.

The reflector rdelined =by Ithe fertile material in the rods 106 and 108 is surrounded, except `for the top thereof, by an outer reflector 166 (FIG. 6) composed principally of fertile material, as for example, bricks or blocks 168 of natural uranium, preferably of high neutronic purity, the principal component of which is Um. The blocks 160 are arranged in annular rows or series spaced radially from the longitudinal axis or center line of the tanks 102 and 104, and the blocks of each row or series are spaced apart, as best seen in FIG. 9. Each row of blocks, as best seen in FIGS. 6 and 8, comprises a plurality of horizontal layers, the spaces between the blocks of the respective layers being verticallyystaggered or misaligned, as seen in FIG. 8. Thus, the reflector 166 is provided with `a multiplicity of passages defined by the space between *the b-locks to afford passage of a reflector coolant such as air or helium underV pressure circulated by inlet `and outlet conduits 170 and 172. This `coolant passes through a reaction chamber 174 containing the tanks 102 and 104 The diameter of each segment is about .394

and the reiiector 166, said chamber being defined by Ia neutron moderating reflector or blanket 176 formed, `for example, of blocks of graphite or beryllium.

The reflector 166 is formed with a smaller number of the radially spaced rows of blocks 168 at the top and bottom of the reflector than at the center thereof, inasmuch as the neutron density is normally greatest in the reflector at that point. Thus, the blocks 168 are sup'- ported by a steel ring 178, the inner surface of which is stepped to accommodate the above-described arrangement of the blocks. The ring 178 is provided with coolant passages 180 and is supported by a biological shield 182 (FIG. 6) mounted on a pneumatic or hydraulic hoist 184, which is, in turn, supported by a pedestal or column 186 adapted to be raised and lowered by any suitable device. such as an hydraulic or screw hoist (not shown) for a punpose hereinafter discussed. The reflector 166 is preferably `about five inches thick at its points of maximum thickness and contains about three and one-half tons of uranium.

The nuclear fission chain reaction within the reactor A is normally controlled by a door 188 in the outer reflector 166, said door preferably being principally composed `of fertile material, as for example, a block of neutronically pure natural uranium. The door is bevelled on the inner surface thereof, as clearly seen in FIGS. 6 and 9, -to define angular paths with the margins of the complementary opening through the yreflector 166, thus `substantially preventing the escape of neutrons through said trated in the drawings.

1-1 opening when the door is in its closed position as is illus- The .door is preferably provided with 'av-plurality of vertical openings 189 (FIG. 9) there- V.through to accommodate flow of the before-mentioned reflector coolant, thus preventing the development of excessive heat within the door due to neutron bombardment thereof. Referring to FIG. 8, it will be seen that the blocks. 168 comprising the layers immediately above and yimmediately below the door 188 are preferably replaced by.-a. pair of annular uranium plates 191, provided with openings or perforations 192 adapted to accommodate .ilow of the reflector coolant therethrough, said plates 191 affording 'a good structural support yfor the blocks at the marginsof tthe-opening through the reflector 166.

Y to rotate the shaft, thus 'actuating the door to the open and closed positions thereof. The reflector 176 is provided with a recess 202 'and 1a vertical groove 204. The recess 202 is adapted to receive the door as the latter is 4actuated to its open position, 4and the groove 204 is adapted toraccommodate vertical movement of the hub `190-and bearing 192 during the elevation and lowering lof the reilector 166, las herein-after described.

-It has been found that when the door 188 is open, vfast neutrons escaping therethrough are scattered through thereaction chamber 174 Iand l'are slowed by elastic collison 'with the atoms of neutron moderator constituting the reflector 176. Thus, the slowed neutrons are unable V,to-penetrate the fertile material constituting the inner and outerV reflectors Iaround the reactor A, and are, for the most part, Vabsorbed by this fertile material or by the wall of lthe outer tank 104. This loss of neutrons reduces the-,neutron reproduction ratio of the reactor A, and :thusfibyadjusting the door 108, this ratio may beA `'adjusted toi-values above and` .below unity, thereby accommodating regulation of theneutron ldensity within the system. It willbe Aunderstoodfthat while one relatively lange door l1'88 is illustrated in the drawings, if desired, any number of, suchgdoors of any suitable size maybe utilized for .thegpurpose of regulating the reaction.

Emergencycontrol may be effected with the door 188 Vclosed,'by dropping the reliector 166 by means of Ithe .hoist i184 or, if desired,v emergency control may be vachievedlby'elevating one, or more of the reactive rods .withinxthe tank 102,y said rods being designated at y10661 in FIGS. `6fand 9; .and being identical with the rods 106,

. except that they are not secured to the bottom wall of the tank i102. Each rod =106a is provided with a Vcoupling V206 (FIG. 6) Iat itsupper extremityfor convenient connection to any,conventionalactuating mechanism l (not shown) supporte-d, for example, by the cover It lwill be understood that from time to time it is 1 desirable to renew the blocks 168 constituting the outer reflector 166, and this is accomplished by actuating the hoist 184 and the column 186 to lower the reflector 166 linto-a: handling 'chamber 208, whereupon a'biological shield 210 is moved, as `for example, by means of wheels 212through a chamber '214 toa position beneath the vrreactor A and above the handling chamber 208. Thereafter-the reflector 166'may lbe disassembled-for the purof recovering the thermally fissionable material formed within 1the blocks 168, Kand a new reflector may beassembled on the ring l178 and may be elevated therewith' into rposition around the reactor A, as shown v40 vkilograms of U235.

Assuming that the Ithermally ssionable segments 124 are'cornposed of vUZ35, the amount required for the abovedescribed reactor is vdetermined by the concentration of this isotope in the segments. Ifv the segments 124 are formed of enriched uranium containing U235 in concentration .of the order of 90% the total amount of enriched uranium-would be A'about 45 .kilognams containingl about Under certain circumstances wherein it is :desirable to raise the number of kilowatts per kilogram of U235, it may be vdesirable to use'this material in more Vdilute-form. For example, enriched uranium `containing U235 in concentration ofthe order of 35% would sustain a reaction in a device such asfabovedescribedvif 100 kilograms of U235 were provided in the reactor. However, Vthe breeding gain and neutron absorption of the coolant is not materially changed by utilizing U235 inV :greater-dilution. It maybe noted that a smaller yquantity of- Pu239 in the reactor Arwould be necessary to sustain-a reaction, all other factors being equal, inasmuch as a -greater Ynumber of neutronsareemitted on the aver- Vage -by Pu239 `atoms during fssioning4 thereof than are emitted Iby U235 atoms.

For an estimation of `the breeding .gain in thermally flssionable material, the geometry of the reactor and the pertinent nuclear constants'must be considered. YThe following vare for the above geometry using reasonable values of the nuclear constants based on actual measurements:

Internal Portion 'of Reactor ln the above tables, are listed in. the iirst column the materials of thel reactor yand in the second column the relative `volumesof these materials in terms of the volume of U235 and.,natural uranium in the iirst and second tables, respectively. The third column indicates the relative number of neutrons absorbed bythe variouscomponents. It is to be noted Vthat in the reactor proper the absorption of neutrons due tothe iron and due to the sodium potassium.in the coolant alloy is negligible compared to' the neutrons absorbed by the U235. true, however, for ythe reflector, and it is believed that the numbers given in the table for the fraction of vneutrons absorbed in theY iron 'and in thealloyy coolant in the Vreflector are high since these calculations assume that all ofthe blanketis iron jacketedfand cooled by the alloy. As above-described, however, the outer reilector V166is so designed that neither iron nor cooling metalY will be present. The other serious loss of Vneutrons from the reactor is ,inf the upper direction where .channels are pro- Vvided `for removinglthe rods lfrom the device. .It may be .suiicientlyfthat the. loss in this direction. can be held no tedthatby lengthening Vthe segments 126 of these rods downto thatestimated forthe other parts of the re- .Y flector. .For the conditions outlined here the breeding gain, Vthat is, the extra few iissionable atoms formed for those consumed, is estimated to be between 0.15 and 0.2.

This is not More explicitly, this means that .for each H235 atom consumed between 1.15 and 1.2 new Pu239 atoms are generated. It should be pointed out that for most purposes concernin-g the chain reaction P11239 is considerably more effective or more valuable than U235, so that more is gained by this conversion than is indicated by these numbers.

It will be apparent from the Idiscussion of the reactor illustrated in FIGS. 6 land 7 that the reactor comprises a central active area containing a iissionable isotope in concentration suicient to be capable of sustaining a fast neutron chain reaction, the central area being surrounded by several concentric areas or blankets. The blankets comprise natural uranium or thorium which not `only absorb neutrons to form a iissionalble isotope but also serve as a fast neutron rreiiector. Other elements are also capable of serving as a fast neutron reflector and may be utilized in one or more of the blankets. Such elements are preferably of relatively high atomic number, that is, above l to minimize neutron moderation. However, elements having an atomic number of 80 'and above which are elements of high scattering cross section such as bismuth, mercury, and lead are preferred as a fast neutron reflecting blanket.

It rwill also be apparent that the reactor shown in the above figures comprises a central active region surrounded by a blanket of thorium or uranium which blanket itself is surrounded by a second blanket comprising a neutron moderator capable of slowing neutrons to substantial neutrons energies such `as caubon, water, deuterium oxide, beryllium or other elements of low thermal neutron crosssection and atomic numbers belo'w about l0. The thorium or uranium blanket not only serves as a fast neutron reflector as above described, but also functions in the nature of a filter or screen for most neutrons of relatively low energies, for `example, 0.01 to l() electron volts. Thus, the fast neutrons leaving the central active `area are either reflected lor captured by the blanket or passed through the blanket into the neutron moderator where they are slowed to low energies, usually to thermal energies. These low energy neutrons are then available for breeding purposes.

Since the critical size of the reactor operating by virtue of thermal neutrons is substantially smaller than the critical size of a reactor operating by virtue of fast neutrons, that is, for example, of the order 100,000 ev. or above, it -is usually desirable to prevent or minimize the return of thermal neutrons into the active area. This is also desirable to minimize the problems yof radioactivity hazards in connection with the coolant system and the like. 'The uranium or thorium blanket serves this function and 4absorbs mo-st of the thermal neutrons which Iare reliected inwardly from the neutron moderator blanket.

It will be understood that other means may be utilized to prevent or minimize the return of slow or thermal neutrons into the centra-l Afast neutron active area, for example, a blanket of cadmium, barium, gadolinium or other isotope having la high neutron capture cross-section for slow neutrons may be provided between the active area and the neutron moderating blanket for this purpose. Likewise, numerous other high neutron absorbers usually having an atomic number below about 80 may be used for this purpose.

It will be understood that the present invention is not restricted to the specific embodiments disclosed or to the theories of nuclear physics set forth, inasmuch as many modifications of these embodiments may be conceived by those skilled in the iart without departing from the spirit of the invention or the scope of the appended claims; and yexperimental data later discovered may modify the theories set forth without affecting the Joperativeness of the invention disclosed.

What is claimed is:

l. A neutronic reactor adapted to breed thermally fissionable material from fertile material comprising a less than critical mass with respect to unmoderated neutrons and a greater than critical mass with respect to neutrons of laverage energy substantially greater than thermal energies of material containing a substantial portion of thermal neutron fissionable material, a liquid coolant comprised of elements selected from the group consisting of sodium and sodium potassium alloys, means for circulating the coolant through the mass, a source of liquid substances comprised of elements selected from the group consisting of lead and lead bismuth alloys, means connecting the source of liquid substances to the mass, and mean 4for inserting the liquid substances into the central` region of the mass, the insertion thereof into the central region of the mass resulting in establishing a greater than critical mass with respect to neutrons of average energy substantially greater than thermal energies, and the withdrawal thereof from the central region of the mass resulting in a less than critical mass whereby a fast neutron chain reaction may be initiated, adapted for optimum breeding of fertile material, and controlled, said entire reactor being substantially free of elements having an atomic number less lthan ll.

2. A neutronic reactor adapted to breed thermally fissionable material from fertile material comprising a plurality of bodies of fertile material enriched in thermally iissionable material in a ratio of from 1.911 to 4:1 disposed in spaced relationships and comprising a less than critical mass with respect to unmoderated neutrons and having a greater than critical mass with respect to neutrons of average energy substantially greater than thermal energies, a liquid coolant comprised of elements selected from the group consisting of sodium and sodium potassium alloys, means for circulating the coolant through the plurality -of bodies, a source of liquid substances comprised of elements selected from the group consisting of lead and lead bismuth alloys, means connecting the source of liquid substances to the plurality of bodies, a plurality of tubes disposed within the plurality of bodies, and means for circulating the liquid substances in controlled quantities through the tubes, whereby a fast neutron chain reaction utilizing in a substantial portion neutrons having an average energy substantially greater than :ther-mal energies may be initiated, adapted to optimum breeding of thermally fissionable material from fertile material and controlled by regulating the quantity of liquid substances in the tubes, said entire reactor being subtantially free of elements having at atomic lnumber less than 1l.

3. A neutronic reactor adapted .to breed thermally lissionable material from fertile material comprising a central core region containing bodies of fertile material enriched in thermally fissionable isotopes in a ratio of from 1.9:1 to 4:1, the core comprised of the total of enriched material being of greater than critical mass with respect to neutrons of average energy substantially greater than thermal energies, fertile material disposed in a layer about the core, a liquid coolant comprised of elements selected from the group consisting of sodium and sodium potassium alloys, means for circulating the coolant through the central core region, a source of liquid substances comprised of elements selected from the group consisting of lead and lead bismuth alloys, means connecting the source of liquid substances to the central core region, means for circulating the liquid substances in controlled quantities through the central core region, and a plurality of rotatable means centered about the periphery of the core comprised of enriched material and fertilematerial in spaced relationship, the rotatable means upon rotation being adapted to transport fertile material into the core region and to transport enriched material out of the core region and intothe region of the layer lof fertile material about the core whereby a. fast neutron chain reaction utilizing in a substantial portion neutrons having an average energy substantially greater than thermal energies may be initiated, adapted for optimum breeding of thermally lissionable material from fertile 

1. A NEUTRONIC REACTOR ADAPTED TO BREED THERMALLY FISSIONABLE MATERIAL FROM FERTILE MATERIAL COMPRISING A LESS THAN CRITICAL MASS WITH RESPECT TO UNMODERARED NEUTRONS AND A GREATER THAN CRITICAL MASS WITH RESPECT TO NEUTRONS OF AVERAGE ENERTY SUBSTANTIALLY GREATER THAN THERMAL ENERGIES OF MATERIAL CONTAINING A SUBSTANTIAL PORTION OF THERMAL NEUTRON FISSIONABLE MATERIAL, A LIQUID COOLANT COMPRISED OF ELEMENTS SELECTED FROM THE GROUP CONSISTING PF SODIUM AND SODIUM POTASSIUM ALLOYS, MEANS FOR CIRCULATING THE COOLANT THROUGH THE MASS, A SOURCE OF LIQUID SUBSTANCES COMPRISED OF ELEMENTS SELECTED FROM THE GROUP CONSISTING OF LEAD AND LEAD BISMUTH ALLOYS, MEANS CONNECTING THE SOURCE OF LIQUID SUBSTANCES TO THE MASS, AND MEAN FOR INSERTING THE LIQUID SUBSTANCES INTO THE CENTRAL REGION OF THE MASS, THE INSERTION THEREOF INTO THE CENTRAL REGION OF THE MASS RESULTING IN ESTABLISHING A GREATER THAN CRITICAL MASS WITH RESPECT TO NEUTRONS OF AVERAGE ENERGY SUBSTANTIALLY GREATER THAN THERMAL ENERGIES, AND THE WITHDRAWAL THEREOF FROM THE CENTRAL REGION OF THE MASS RESULTING IN A LESS THAN CRITICAL MASS WHEREBY A FAST NEUTRON CHAIN REACTION MAY BE INITIATED, ADAPTED FOR OPTIMUM BREEDING OF FERTILE MATERIAL, AND CONTROLLED, SAID ENTIRE REACTOR BEING SUBSTANTIALLY FREE OF ELEMENTS HAVING AN ATOMIC NUMBER LESS THAN
 11. 